Current therapeutic approaches to Crohn's Disease (corticosteroids, neutralization of TNF-?) rely upon broad-spectrum immunosuppression, an approach that is not uniformly effective and is frequently associated with major side-effects. To address this market gap, Radikal Therapeutics is developing a novel soluble receptor decoy receptor fusion protein (hR-421) that induces immune tolerance in antigen (Ag)-specific activated T- cells. Constructed from Ig-Fc and the second ecto domain of the CCR5 receptor, hR-421 binds and neutralizes all 3 CCR5-binding ligands (chemokines MIP-?, MIP-?, RANTES) and prevents their binding and activation of CCR1, CCR3, and CCR5. In contrast to CCR5 inhibitors, mR-421 (the murine homologue of hR-421) blocks CCR5-independent pro-inflammatory pathways and is effective in CCR5 knock-out mice. Therapy with mR-421 profoundly suppresses experimental allergic encephalomyelitis (EAE), a classic autoimmune model system, even when treatment is initiated after disease onset. Moreover, Ag-specific effector Th1 cells isolated from EAE donors treated in vivo with mR-421 produce substantially less pro-inflammatory cytokines and suppress EAE in adoptive transfer experiments. The purpose of the current application is to extend the protective effects of the R-421 technology beyond the preliminary findings in neuroinflammation and establish its potential benefit in a clinically-relevant model of Crohn's Disease. The proposed studies are specifically designed to test the hypothesis that mR-421 attenuates preexisting colitis via its suppression of pro-inflammatory cytokines in the intestinal mucosa. Specific Aims: Establish the pharmacodynamic profile of mR-421 in mice utilizing 1) a dextran sulfate sodium (DSS) model and 2) a dintrobenzenesulfonic (DNBS) model of intestinal inflammation Treatment with mR-421 (3, 10, and 30 mg/kg/day QD IP), an irrelevant IgG control (10 mg/kg QD IP), dexamethasone (1 mg/kg QD IP), or anti-murine TNF-? (10 mg/kg QD IP) will begin 7 days after the start of oral DSS or intrarectal DNBS exposure, a timepoint of significant ongoing colitis. A sham group without exposure to DSS or mR-421 will be utilized as a control. Animals will be monitored for body weight and clinical signs of illness: hunched-over appearance, weight loss, loose stools/diarrhea, and bloody stools. After 10 days of therapy, the intestine will be analyzed for biochemical, immunohistochemical, and morphologic parameters indicative of inflammatory injury. Progression to the Phase 2 SBIR will require that with mR-421: 1) dose-dependently ameliorates tissue injury, as demonstrated by > 50% reductions in all of the following injury parameters: lipid peroxidation, neutrophil (PMN) infiltration, peroxynitrite (ONOO-) production, poly(ADP-ribose)polymerase formation, gross and histologic injury scores, and 2) demonstrate a clear mechanism of action, as shown by suppression of mucosal pro-inflammatory cytokines and chemokines in the DNBS and DSS models. All of the above effects of mR-421 must be superior relative to the irrelevant IgG control group and at least non-inferior relative to therapy with an anti-murine TNF-? mAb and dexamethasone (at p<0.05). PUBLIC HEALTH RELEVANCE: Crohn's Disease remains recalcitrant to existing therapies, with a high percentage of patients enduring recurrent bouts of inflammation and intestinal damage. We are developing a novel drug that specifically blocks the inflammatory process in this condition. We will test this agent in a series of clinically-relevant animal models of autoimmune colitis.